Liquid crystal display devices are used not only as large-sized television sets, but also as small-sized display devices, e.g., the display sections of mobile phones. Liquid crystal display devices of the TN (Twisted Nematic) mode, which have often been used conventionally, have relatively narrow viewing angles. In recent years, however, liquid crystal display devices with wide viewing angles have been produced, e.g., the IPS (In-Plane Switching) mode and the VA (Vertical Alignment) mode. Among such wide-viewing angle modes, the VA mode is adopted in a large number of liquid crystal display devices because of an ability to realize a high contrast ratio.
As one kind of VA mode, the MVA (Multi-domain Vertical Alignment) mode is known, under which a plurality of liquid crystal domains are created in one pixel region. An MVA-mode liquid crystal display device includes alignment regulating structures provided on the liquid-crystal-layer side of at least one of a pair of opposing substrates, between which a vertical-alignment type liquid crystal layer is interposed. The alignment regulating structures may be linear slits (apertures) or ribs (protruding structures) that are provided on electrodes, for example. The alignment regulating structures provide alignment regulating forces from one side or both sides of the liquid crystal layer, thus creating a plurality of liquid crystal domains (typically four liquid crystal domains) with different alignment directions, whereby the viewing angle characteristics are improved.
Also known as a kind of VA mode is the CPA (Continuous Pinwheel Alignment) mode. In a generic liquid crystal display device of the CPA mode, pixel electrodes of a highly symmetrical shape are provided, and on a counter electrode, protrusions are provided corresponding to the centers of liquid crystal domains. These protrusions are also referred to as rivets. When a voltage is applied, in accordance with an oblique electric field which is created with the counter electrode and a highly symmetrical pixel electrode, liquid crystal molecules take an inclined alignment of a radial shape. Moreover, the inclined alignment of the liquid crystal molecules are stabilized due to the alignment regulating forces of side slopes of the rivets. Thus, the liquid crystal molecules in one pixel are aligned in a radial shape, thereby improving the viewing angle characteristics.
Unlike in a TN-mode liquid crystal display device in which the pretilt directions of liquid crystal molecules are regulated by alignment films, alignment regulating forces are applied to the liquid crystal molecules by linear slits or ribs in an MVA-mode liquid crystal display device, so that the alignment regulating forces to the liquid crystal molecules within a pixel region may differ depending on the distance from a slit or rib, and differences may occur in the response speeds of the liquid crystal molecules within the pixel. Similarly, also in the CPA mode, differences may occur in the response speeds of the liquid crystal molecules within the pixel, and the differences in response speed will be more outstanding as the pixel electrode increases in size.
In the generic VA mode, liquid crystal molecules are aligned in the normal direction of the principal face of an alignment film in the absence of an applied voltage, and when a voltage is applied across the liquid crystal layer, the liquid crystal molecules are aligned in a predetermined direction. On the other hand, in order to improve the response speed of the VA mode as such, conducting a treatment called Polymer Sustained Alignment (hereinafter referred to as the “PSA treatment”) is under study (see Patent Documents 1 and 2). In a PSA treatment, while applying a voltage across a liquid crystal layer having a small amount of polymerizable compound (e.g., a photopolymerizable monomer) mixed thereto, polymerization of the polymerizable compound is effected, whereby the alignment state of the liquid crystal molecules is sustained (stored), and a pretilt is conferred such that the liquid crystal molecules are inclined from the normal direction of the principal face of the alignment film even in the absence of an applied voltage.
The liquid crystal display devices of Patent Documents 1 and 2 include an electrode having a minute stripe pattern such that, when a voltage is applied across the liquid crystal layer, the liquid crystal molecules are aligned in parallel to the longitudinal direction of the stripe pattern. In this state, if ultraviolet light is radiated so as to allow the alignment state of the liquid crystal molecules to be sustained (stored), the liquid crystal molecules will thereafter be inclined in the pretilt azimuth from the normal direction of the principal face of the alignment film, even when voltage application is stopped. In this manner, a pretilt is conferred to the liquid crystal molecules in the absence of an applied voltage, thus obtaining an improved response speed. Applicability of a PSA treatment in not only the aforementioned VA mode but also in other modes, e.g., the IPS mode or the TN mode, is also under study (Patent Document 3).